This invention relates to a vapor generating system and, more particularly, to a sub-critical or super-critical once-through vapor generating system for converting water to vapor.
In general, a once-through vapor generator operates to circulate a pressurized fluid, usually water, through a vapor generating section and a superheating section to convert the water to vapor. In these arrangements, the water entering the unit makes a single pass through the circuitry and discharges through the superheating section outlet of the unit as superheated vapor for use in driving a turbine, or the like.
These arrangements provide several improvements over conventional drum-type boilers, and although some problems arose in connection with early versions of the once-through generators, such as excessive thermal losses, mismatching of steam temperature, the requirement for sophisticated controls and additional valving during startup, these problems have been virtually eliminated in later generation systems.
For example, the system disclosed in U.S. Pat. No. application Ser. No. 713,313 filed on Aug. 10, 1976, now U.S. Pat. No. 4,099,384, and assigned to the assignee of the present invention, includes a plurality of separators disposed in the main flow line between the vapor generating section and the superheating section and adapted to receive fluid flow from the vapor generating section during startup and full load operation of the system. This arrangement enables a quick and efficient startup to be achieved with a minimum of control functions, and without the need for costly valves. Also, the turbines can be smoothly loaded at optimum pressures and temperatures that can be constantly and gradually increased, without the need of boiler division valves or external bypass circuitry for steam dumping. Also, according to this system operation can be continuous at very low loads with a minimum of heat loss to the condenser.
In the latter arrangement, the walls of the furnace section of the generator are formed by a plurality of vertically extending tubes having fins extending outwardly from diametrically opposed portions thereof, with the fins of adjacent tubes being connected together to form a gas-tight structure. During startup the furnace operates at constant pressure and super-critical water is passed through the furnace boundary walls in multiple passes to gradually increase its temperature.
In this arrangement the upper portions of some of the tubes forming the rear boundary wall of the furnace section are bent out of the plane of the lateral wall and upwardly to form a gas screen for the passage of gases from the furnace section to a heat recovery area disposed adjacent the furnace section. However, this design requires precision and tedious fabrication which is difficult and time-consuming and, therefore, expensive.